The present invention relates to amorphous metallic materials, and, more particularly, to the use of materials that transform from a crystalline to an amorphous state under a wearing action.
Wear occurs by the gradual removal of material at a surface exposed to a wearing environment. Reduction of the effects of the wear of materials is a problem of great significance, as each year the direct and indirect costs resulting from wear amount to billions of dollars. Many techniques have therefore been devised to reduce the wear of articles to acceptable levels, including the development of the materials having greater inherent wear resistance, the selection of designs intended to minimize wear, the use of lubricants to reduce surface contact, and the placing of wear-reistant coatings on parts to resist the damage done by wear.
In one approach to wear-resistant coatings, very hard materials are placed on the surfaces of the parts being protected. The hard coatings have higher wear resistance than do the substrates upon which they are coated, thereby acting to reduce the total wear experienced. As an example, a thin layer of a hard, wear-resistant material such as a tungsten carbide-cobalt composite may be bonded onto the exposed surfaces of an article before the article is placed into a wear-including environment. In another example, other kinds of hard particles, such as chromium carbide, may be dispersed throughout a coating matrix, which itself can be hardened. The dispersed hard particles resist frictional wear, but such coating systems suffer from the loss of particles due to wearing of the matrix and undermining of the particles.
In one promising approach to surface protection, it has been found that some metallic materials are extremely wear resistant, moderately ductile, tough and resistant to corrosion when in an amorphous state. Certain materials can exist in both the amorphous and nonamorphous (or crystalline) states, and exhibit the improved wear-resistant properties when in the amorphous state. Most solid metals normally exist in the crystalline state, and special processing is required to place them into the amorphous state. The amorphous state may be produced by any of several techniques, such as rapid quenching from the liquid state, ion implantation, or electrodeposition in some instances. Amorphous materials have regions of no short range or long range order, and also have no grain boundaries.
To enjoy the benefits of the high wear resistance offered by amorphous materials using present preparation techniques, the amorphous materials must be prepared and then joined to the surface to be protected, must be very carefully deposited onto the surface in the amorphous state, or must be otherwise specially prepared. Further, some surface areas of an article wear more rapidly than others, and the usual practice is to apply conventional protective coatings more thickly in such areas. However, controllable thicknesses of amorphous coatings cannot readily be applied, because of inherent limitations in achieving an amorphous as-deposited structure more than a few thousandths of an inch thick.
Thus, while amorphous materials offer great promise for use as wear-resistant coatings on parts, it is difficult to achieve their benefits because of the problems encountered in preparing the amorphous material and then attaching it to the surface, or preparing the amorphous material in place on the surface. As with all coatings applied as protective layers, if the thin amorphous coating is worn away in a location of particularly intense wear, the wearing environment may penetrate under the coating. A localized penetration of the coating can grow in lateral extent rapidly, so that adjacent portions of the coating are undermined and the coating flakes away. There is then a very rapid increase in the rate of damage, so that failure of a part thought to be protected can occur catastrophically, and between opportunities for inspection.
To achieve its benefits, great care must be taken to obtain the amorphous structure over a large surface area and particularly in areas which may be relatively inaccessible, as many coatings are much less wear-resistant after crystallization, as compared with their amorphous state. Yet, as the wear develops it may be discovered that only a relatively limited portion of the surface requires the greater wear resistance provided by the material in its amorphous state. In such circumstances, a great portion of the effort devoted to obtaining an acceptable amorphous structure over the entire surface is wasted, as only the limited portion is subjected to severe wear. On the other hand, the relatively limited portions where wear is greatest may be located in inaccessible places such as behind projections on the surface or at reentrant corners, so that application of the amorphous coating is most difficult in the area of the greatest need.
There is therefore a need for an improved process and material for protectin the surfaces of articles to obtain the benefits of the highly wear resistant amophous materials. Such a process should be economical, usable over both broad surface areas and in relatively inaccessible locations, and produce a structure which is resistant to catastrophic failures such as resulting from penetration of the areas subjected to the most intense wear. The present invention fulfills this need, and further provides related advantages.